Chromium recovery



Aug. 11, 1942.

INVENTOR ATTORNEYS Patented Aug. il, i942 MMWR@ STATES $TENT @FFMECHROMIUM RECOVERY Marvin i. Udy, Niagara Falls, N. Y.

Application August 15, 1Mo, Serial No. 352,746

(Cl. 'i5-84) 7 Claims.

This invention relates to chromium metallurgy and has for an object theprovision of certain improvements in chromium recovery processes. Moreparticularly, the invention contemplates the provision of improvedmethods of concentrating chromium-bearing materials to obtain productshigh in chromium and low in iron and products containing relativelysmall amounts `of gangue materials such as alumina and silica.Theinvention further contemplates the treatment of ma.- terialscontaining spinels comprising ferrous oxide, alumina and chromlc oxideto produce altered spinel products containing ferrous oxide alumina inproportions, relatively to chromic oxide, smaller kthan in the spinelsof the original chromium-bearing material. Another object of 'theinvention is toY produce novel and improved chrornium-bearingy productso the nature of artificial spinel type minerals and altered mineralconcentrates. The invention also contemplates the production of improvedchromium-bearing materials and improved composite reagents ior use inproducing ferrochromium and for use in incorporating chromium vin ironand steel, the provision of improved methods of producingchromium-bearing materials and improved composite reagents for such usesand the provision of improved methods of producing ferrochromium andchromium-bearing iron and steel products. invention further contemplatesthe provision of composite reagents of various compositions suitable foreffective and eicient use in operations characterized as to type byperformance in electric furnaces, combustion furnaces, foundry ladles,crucibles and other types of equipment.

The invention is based on my discovery that chromium-bearing materialsof the nature of chromite ore can be made more yamenable to treatmentfor the recovery of the contained chromim in useful form by fusing theore with lime (CaO, calcium oxide) or magnesia (MgO, magnesium oxide) ora mixture of lime and magnesia in controlled amounts or proportions. Ihave discovered, also', that additional advantages 'can be obtained whenthe chromite ore is fused With lime in amounts such as to produce afused product having the property or characteristic ofselfdisintegration upon cooling.

By fusing chromium-bearing materials containing minerals of the spine]type and thereafter subjecting the product of the fusion treatment to a'concentration treatment in accordance with the invention, I obtainconcentrates comprising altered spinel type minerals which alteredminerals contain chromic oxide in greater proportions .by weight andchromium and iron in higher ratios of chromium to iron than the spineitype minerals of the original chromium-bearing materials. The alteredmineral products of my in- 'vention may .be suiiciently free of ferrousoxide and alumina to approximate true magnesium chromite in compositionor they may contain ferrous oxide and alumina in controlled relativelysmall amounts. in practicing my invention, i prefer to form and recoveraltered mineral concentrates containing not less than about nity percent. -by weight of chromic oxide and chromium and iron in proportionsto give a ratio of chromium to iron above three' to one (3:1). l haveproduced altered mineral concentrates containing more than sixty percent. (60%) and even as high as 'seventy-eight per cent. (78%) chromlcoxide by weight. All concentrates contained iron and alumina inproportions relatively to chromic oxide lower than contained in thespine'. type minerals of the original chromium-bearing materials.Chromite ores constitute the principal raw lmaterials from whichchromium is recovered, and,

therefore, the inventionl will .be described herefor example, in theproduction of chromiumbearing iron and steel products. The invention iscapable of producing concentrates free of car- -bon and suchconcentrates may be employed to great advantage in the production offerro-chromium and chromium-bearing iron and steel products. I employthe concentrates for producing the composite reagents of my inventionwhich reagents are particularly well adapted for use in producingferrochromium and in vproducing chromium-bearing iron and steelproducts.

The composite reagents of my invention comprise mixtures containing thealtered mineral concentrates, or iron land chromium oxide-bearingproducts resulting from oxidation of the concentrates, and one or moresolid, non-carbonaceous reducing agents such, for example, as calcium,aluminum or siliconor an alloy of calcium, aluminum or silicon with oneor more other elements: for example, calcium silicide, ferrosilicon,ferrochrome silicon. aluminum silicide or ierroaluminum silicon. Theoxide-bearing ma.- terial and the non-carbonaceous reducing agentpreferably are finely divided and intimately mixed. The degree ofsub-division and intimacy of mixing in any case will depend to someextent at least on the particular manner in which the composite reagentis to be used. When the heatdeveloped as the result of reaction betweenthe components of a composite reagent is to be relied upon largely orentirely for promoting and continuing the reaction and for melting thereaction products, the degree of subdivision and intimacy of mixing ofthe components preferably are such that every particle of reduciblematerial contained in the mixture is in direct and substantiallycomplete contact with particles of reducing agent. I have found thatsuch intimate contact requires a degree of comminuation such that alarge proportion of the oxide-bearing material and the reducing agentconsists of -particles sufficiently small to pass a 1D0-mesh screen(Tyler series) and grinding of the materials in contact, or together.When substantial amounts of heat are available, as, for example, whenthe composite reagents are to be treated in electric furnaces, thecomponents need not be so finely divided and mixing need not be sointimate.

When the composite reagents are to be employed in operations employingelectric furnaces vide about the theoretical amount of silicon forcombining with the available oxygen. Ii excess silicon is used, theexcess will enter the metal produced, control or this being effected inthe use of reducible materials, high lime slags, and high temperatures.

'I'he invention may be employed in the treatlment of chromite ores ofany grade, but it is of particular importance with respect to the matterof utilizing the so-called low-grade ores, that is, ores containing lowpercentages of chromium and ores whichcontain iron and chromium inratios of iron to chromium too high to permit them to be utilized forthe recovery o! chromium for industrial uses in processes of the typeemployed heretofore for chromium -recovery. The

, invention presents a solution of the problemroi Where ample suppliesof heat are available, thecomposite reagents may contain the alteredmineralconcentrate in the unoxidized condition in which it is recovered.When sumcient extraneous heat to effect reduction of the chromium of theconcentrate by means of the reducing agent is not available, theconcentrate, or a portion of it, preferably is subjected to an oxidationtreatment to incorporate oxygen in a form in which it is available forreaction with the reducing agent to supply heat for effecting reductionand melting the products of the reduction reactions.

The products of oxidation are admirably suited for use in formingexothermic mixtures for use in open hearth steel practice. Oxidation isreadily controlled to form` products containing suilicient oxygen tosupport combustion of reducing agents, which may be substantiallycomplete if desired. The ferrous oxide of the altered mineralconcentrate isv converted readily to ferrie oxide (FezOc), and oxidationmay be so controlled as to convert any desired proportion of the chromicoxide to chromic anhydride (CIOs) Depending upon the conditions underwhich the various composite reagents are to be employed,

they may contain additional oxygen-containing substances to provideadditional heat for melting the metal and slag produced.

`Any of the common oxidizing agents such, for example, as sodiumnitrate, sodium. chlorate and manganese dioxide may be employed forpromoting oxidation of silicon with the resultant production ofadditional heat. Such oxidizing agents are required, for example, whenthe reaction mixtures contain large amounts of inert materials such asslag-forming materials, and they are employed in amounts (together withthe necessary reducing agent) suilicient tol produce venough heat tomelt the metal and slag produced and give the slag the desired iiuidity.The addition of such oxidizing agents requires additional silicon, whensilicon is employed for reduction, and the total amounts of silicon andavailable oxygen are so proportioned as to proutilization oi suchso-called low-grade ores, a solution of major importance to countriesrequiring chromium for industrial uses, but having available onlydeposits of low-grade ores. The invention provides a further speciiicadvantage of importance by permitting the use of combustion iurnaces inpreliminary fusion treatments to condition chromite ores ior subsequentconcentration treatments with recovery of high grade vconcentrates.

The chromium of chromite ores usually is pres- "ent in the spinel typemineral, MgOFeO (CrnOs A1203) and the gangue materials associated withthis mineral in the ore usually contain silica (SiOz), magnesia (MgO)and alumina (A1203). I have found that, in some rare instances, notablyin some Canadian deposits, magnesia in the ore as mined is present inthe gangue materials combinedA with carbon dioxide as magnesiumcarbonate, but usually the magnesia of the gangue materials is all orlargely combined with the silica and alumina, probably asmagnesiumaluminum-silicate.

In the usual chromite ores, the gangue materials are so widelydisseminated and so intimately lintermixed with the spinels thatconcentration with recovery of substantially pure spinel particles isvirtually impossible. It appears that separation of relatively purespinel particles, if such separation is at all possible, might require:tine grinding ofthe ore toa degree such as to destroy the properties ofthe components of the ore by virtue of which they might be selectivelyseparable in known concentration processes. Fusion of the ores inaccordance with my invention appears to result in concentration of thespinels into relatively large and relatively pure particles, withincrease in speciiic gravity. The spinels and associated ganguematerials are so altered that they may be separated readily by ordinaryconcentration methods, with the production of tailing products low inchromium oxide and concentrate products high in chromium oxide.

My researches and experiments have indicated that when a chromite ore isfused with nme presence of the lime with the production ofcalcium-,aluminum-silicate. The tendency of the alumina (A1203) is todivide between the silicate and chromite portions with a somewhatgreater venting introduction of such components into the altered spineltype minerals produced. The preferred charges of the invention (for thefusion treatments) consist of chromite ore and a suitable basiccomponent such as lime. When a disintegrating product-is sought, silicamay be included if necessary, and,y if fractional reduction of displacedferrous oxide is desired, a reducing agent may be included in 'thecharge to be treated.

Magnesium oxide may be employed instead of calcium oxide to eiect directdisplacement of the ferrous oxide from the spinel type mineral, but Iprefer to employ calcium oxide or calcium oxide and magnesium oxidetogether, rather than magnesium oxide alone because of advantages infurnace operating characteristics and in the properties of the alteredore resulting from the use of lime (calcium oxido). The invention willbe described' hereinafter more particularly with respect to the use oflime in the fusion treatment. '.llie lime and magnesia, or either, maybe employed as such, that is, as calcium oxide and magnesium oxide, oreither or both may be employed in the carbonate or other form which willbe `converted to the oxide form in the Jlusion treatment.

Fusion ci a charge containing lime and chromite ore may be carried outat a relatively low temperature ci incipient fusion at which a productin the form of clinker will be produced, or, fusion of the charge may becarried out at a relatively high temperature at which a product in theform -of a molten bath will be produced.

Fusion treatments of the invention may be oarried outl in any suitabletype of furnace or heating equipment. For heating charges totemperatures of incipient fusion, I prefer to employ combustion heatedfurnaces of the type Aof cement kilns, and, for heating charges tomelting temperatures, I prefer to employ the submerged arc type electricfurnace. Combustion heated equipment of the type of cupolas and blastfurnaces may be employed if desired.

It the lime is present in the charge in an amount less than orsubstantially in excess of the amount required to form di-calciumsilicate, the product of the fusion treatment may not possess theproperty of self-disintegration. Also, if the amount of silica presentin the charge is not sufcient to permit the formation of a substantialamount of cli-calcium silicate, the product of the fusion treatment maynot be self-disintegrating, but, on the contrary, may set from the fusedstate as a hard, vitreous mass when the fusion treatment is carried outat a temperature sufciently high to form a molten product, or as hard,vitreous clinkers when the fusion treatment is carried out at arelatively low temperature of incipient fusion.

In practicing my invention, Iomay employ charges of any suitablecompositions, and I may produce either disintegrating ornon-disintegrating fused products. I prefer to employ 'grating productis sought.

charges containing lime and silica in proportions such thatself-disintegrating products will result .from the fusion treatments. Ihave found charges made from ore containing about six percent. (6%) ormore of silica to be satisfactory for vproducing self-disintegratingproducts when suitable amounts of lime also are included in the charges.Chromite ores usually contain suilicient silica, but silica may be addedl15o-cffect the production. of self-disintegrating products when oresdecient in silica are undergoing treatment. To insure the production ofa disintegrating slag, a charge to be subjected to a fusion treatmentshould contain a total amount of lime (CaO) or magnesia. (MgO) or bothat least equivalent molecularly to the chromic oxide (CrzOs) of thecharge plus an amount of limev (CaO) equivalent to two molecules of lime(CaO) for each molecule of silica in the charge. Usually, the amount oflime included in a charge to be fused should be in excess of the amount'theoretically required to produce di-calcium sillcate with the silicapresent when a self-disinte- The necessity for using a charge containingsuch an excess of lime probably results from the fact that a portion ofthe lime enters into various combinations with the magnesia, alumina,chromium oxide and ferrous oxide present in the charge arid is notavailable to form til-calcium silicate. Usually, an amount of lime aboutten per cent. (10%) to forty or fty per cent. (4.0 or 50%) in excess ofthe amount theoretically required to form dicalcium silicate with thesilica in a charge will result in the production of a sef-disintegratingproduct through fusion of the charge. The amount oi lime (Cao) whichshould be present in a charge to produce a sel-disintegrating productthrough fusion can be gauged or estimated roughly as an amount about tento fteen per cent. (l0 to 15%) in excess of that'equivalent to theferrous oxide (Feo) in the spinel type mineral of the chromite ore ofthe charge.

When a charge comprising chromite ore is subjected to a fusion treatmentin the presence of lime, the ore is altered or converted from aSubstantially acid-insoluble product to a product which may be dissolvedor broken down with acids, and, according to one aspect of my invention,I utilize the change in susceptibility to attack by acids as an aid tothe recovery of high grade concentrates by means of suitable acidtreatments. The degree of conversion depends upon the amount of limeemployed. With a small amount of lime, the impurities associated withthe spinel type mineral of the ore became susceptible to attack byacids, and, as the amountof lime employed is increased, the alumina ofthe spinel type mineral is displaced or removed and enters intocombination with the excess lime. As the amount of lime is increased,there is also a tendency for the chromium oxide to become acid soluble,and, conceivably, the entire chromic oxide content of an ore can berendered soluble through the use of lime in suiicient quantity. 'I'heprogressively increasing susceptibility of the ore to attack by acidswith increasing solubility of the chromic oxide through the use ofprogressively increased amounts of lime may be attributed to progressivealterationv of the spinel type mineral of the ore with lime rs't causing(indirectly) displacement of the ferrous oxide, and magnesiasubstitutingfor the displaced ferrous oxide, then with alumina beingabstracted more and more from the spinel type mineral ungravities(gravity concentration methods,

til the spinel type mincral approaches or reaches the form of a truemagnesium chromite (spinel) contaminated to some extent with smallamounts of ferrous oxide, calcium chromite and alumina, and, ultimately,with decompositions of the magnesium chromite spinel and the productionof mineral bodies of indeterminate compositions. The altered spinel typemineral crystallizes in the fused product in particles of sufficientmass and suiliciently free of contamination with other minerals orcompounds contained in the fused product that they may be separated andrecovered from such other minerals or compounds by concentration methodsemploying the principle of separation by virtue of differences inspecific including for example, tabling, hindered settling and hydraulicclassification) and by concentration methods employing the principle ofseparation by virtue of differences in aiiinities of chemical reagents(flotation concentration methods). The spinels and other minerals andcompounds associated therewith in the fused products have differentmagnetic susceptibilities and, therefore, magnetic separation methodsmay be adapted for separation and recovery of the spinels.

In preparing chromium-bearing materials by means of fusion treatmentsfor subsequent treatment by concentration methods, it is advisable toemploy lime or lime and magnesia in such amounts and proportions as toeffect the desired alteration of the spinel with minimum conversion ofthe chromic oxide to the acid-soluble condition consistent witheconomical operation under the economic conditions peculiar to thelocality in which concentration is to be carried out.

In practicing the invention, fusion of a charge comprising chromite oreand lime may be carried out under neutral, oxidizing or reducingconditions. If fusion of the charge is carried out under controlledreducing conditions, some, or even substantially all, of the iron of thedisplaced ferrous oxide may be reduced preferentially to the metallicstate, leaving a large proportion, or even substantially all, of thechromium unreduced. When the fusion treatment is carried out at atemperature such that the charge becomes molten, the metallic iron andthe unreduced chromium will be contained in separable metal and slaglayers, respectively, and they may beseparated by procedures well knownin the metallurgical art. When the fusion treatment is carried out at arelatively low temperature of incipient fusion and under reducingconditions, the metallic iron formed will be distributed in the form ofsmall particles throughout the residual non-metallic material containingthe unreduced chromium. Separation of the metal particles from thenon-metallic material may be accomplished by any suitable means.

In a preferred process of my invention, I simply heat the chargecomprising spinel-bearing material and lime to a temperature ofincipient fusion (cement kiln temperaturas oi about 1350 C. to 1400 C.)under non-reducing conditions to form an altered ore product containingall of the iron and chromium of the original chromium spinel-bearingmaterial. Preferably, the composition of the charge subjected to thefusion treatment is so adjusted that the resulting product isself-disintegrating. The self-disintegrating product of the fusiontreatment is digested with acid to dissolve its soluble components, andthe insoluble residue is subjected to a gravity or notationconcentration treatment to recover a .accenna concentrate containingchromic oxide in higher concentration than in the original ore.

In another preferred process of my invention employing reducingconditions during the fusion treatment, I practice controlled reduction,reducing the major portion of the iron of the displaced ferrous oxidemajor portion' of the chromium of the ore. I thus obtain a metallic ironproduct rela-.-

tively low in chromium and a non-metallic altered ore productbenenciated with respect to chromium by virtue of an increase in theratio of chromium to iron resulting from the Preferential or selectivereduction of the iron of the original ore. In practicing controlledreduction, I prefer to `leave -unreduced a subsantial amount ofdisplaced ferrous oxide, particularly when an acid treatment is to beemployed subsequently to decompose the fused product, and I have foundthat ferrous oxide disseminated throughout the fused material aids indecomposition by acid by permitting penetration of the acid between thegrains or crystals of splnel.

When the fusion treatment is carried out at a temperature such that thecharge becomes molten, controlled reduction may be practiced byincluding in the charge the amount of carbonaceous material required toreduce the amount of iron sought to be reduced. When the fusiontreatment is carried out at a relatively low ternperature of incipientfusion, controlled reduction is rather simple; iron is reduced readilyat such low temperatures Whereas chromium can not be reduced to anysubstantial extent at such temperatures.

Controlled reduction may be carried out with any common reducing agentsuch as solid or gaseous carbonaceous materials, hydrogen, silicon,aluminum or a silicide, or, metallic chromium may be employed as areducing agent for-the ferrous oxide to simultaneously enrich the fusedproduct in chromium. Reduction by means of metallic chromium must becarried out with the metallic chromium and the chromium-bearing materialundergoing the reduction treatment in molten condition, and, in thistype of operation, a relatively low-grade ferrochromium product may beused advantageously to supply the metallic chromium. Silicon-bearingreducing agents may be employed advantageously for controlled reductionof .the displaced ferrous oxide when a self-disintegrating fused productis sought and when the chromium-bearing material undergoing treatmentrequires additional silica to form the amount of di-calcium silicaterequired to effect disintegration. When metallic chromium is employed asa, reducing agent, the charge. composition should be adjusted to includetherein suiilcient magnesium oxide to combine with the chromic oxideresulting from reduction of the ferrous oxide as well as with thechromic oxide .of the chromium-bearing material undergoing treatment.Metallic chromium and elemental silicon may be used together as reducingagents advantageously in the form of ferrochrome silicon which may beproduced by fusing the metallic iron product of the controlled reductionoperation with coke and silica to reduce the silicon of the silica.

In the foregoing discussion, I have advanced a theory, which seems to besupported by experimental results, in an effort to explain the mechanismof alteration of chromite ore resulting from fusion of .the ore withlime, but it is to be underand leaving unreduced the stood that I do notwish to be bound or limited by theoretical considerations. The resultsofmy researches and experimentations denitely establish the fact ofalteration resulting from fusion of chromite ore with lime. As theresult of such fusion treatments, the iron of the ferrous oxide ofchromite ore becomes more amenable to selective or preferentialreduction by common reducing agents; the chromium of the chromic oxideof chromite ore becomes more readily aridi...- able; the ferrous Aoxideof the chromite ore becomes more susceptible to attack by acids; and thechromic oxide becomes more amenable to recovery by ordinaryconcentration methods.

In concentrating chromite ore to produce a high-chromium concentrate inaccordance with my invention, I may as indicated above, fuse the oreunder such conditions as to produce a selidisintegrating product orunder conditions such as to produce a non-disintegrating product, and imay carry out the fusion treatment under reducing conditions to producea beneciated a1- tered ore product or under non-reducing conditions toproduce 'merely an altered, ore product containing all of the iron andchromium present in the original ore. d self-disintegrating fusedproduct ofthe invention readily disintegrates to form particles whichare largely small enough to cass a (i5-mesh screen, and the entireproduct nay be ground readily to form particles small enough to pass aG-mesh screen.

When a self-disintegrating fused -product is aroduced, I may concentrateby means of acid nethods, gravity concentration methods (such as vaterconcentration or air separation) flotation :oncentration methods or anysuitable combina- .ion of two or more or such methods. When ravityconcentration is employed, I may rereat the concentrate obtained withacid to iurher eliminate associated gangue materials. *Vhen anon-disintegrating product is formed, I nay grind the product andsubject the ground roduct to a gravity concentration treatment or o aflotation concentration treatment or both, `ut I prefer to concentratethe ground product y means of an acid treatment which may be folwed by agravity or notation concentration reatment.

In processes involving melting of the charges, he original chargematerials may be employed 1 any suitable particle sizes. In processesinolving sintering (incipient fusion) of the harges, the chargematerials preferably are emloyed in the form of particles one-quarterinch nd smaller in size. When a se -disintegrating roduct is to beformed by sintering (incipient ision) I prefer to employ chargematerials consting essentially of particles minus 65-mesh in ze or, moredesirably, minus 10U-mesh in size. the components of the charge are tobe emloyed in the form of particles of dilerent sizes L the productionof self-disintegrating altered .-e products by fusion, it is advisableto employ 1e lime in the more finely divided condition imaller particlesize). (Screen siz'es referred to l'this application are based on theTyler series.) In treating any of the products oi' fusion treatents, Imay rst employ an acid treatment or i alkali treatment to decompose thefused prod- :t and follow this treatment with a gravity or :tation orequivalent concentration treatment recover a concentrate containing thealtered inel type mineral, or, I may rst employ a 'avity or notation orequivalent concentration eatment to recover a concentrate containing thealtered spinel type mineral and subsequently subject the concentrate toan acid cleaning or ICaustic soda or sodium carbonate may be employedfor decomposition.

In treating the products of fusion treatments with acid, and, incleaning concentrates resulting from various types of concentrationtreatments, I may employ any suitable acid in any suitable concentrationcapable of eecting the degree of decomposition sought to beaccomplished. I have found hydrochloric acid and sulphuric acid to beentirely suitable for accomplishing edective decomposition of the fusedproducts and concentrates. Aqueous solutions containing about ten tothirty per cent. (l0 to 30%) hydrochloric acid (HC1) or sulphuric acid(HaSOi) can be employed to produce good results at temperatures rangingfrom ordinary atmospheric temperatures to bolling temperatures.Sulphuric acid treatments may be followed by roasting or fumingtreatments at elevated temperatures to promote more edectivedecomposition oi the fused lproducts or concentrates and subsequently todecompose or break down sulphate compounds produced.

The following examples illustrate the production and recovery of highchromic oxide (CrzOs) spinels with low silica and alumina produced bycarrying out fusion and concentration treatments in accordance with myinvention, employing chromite ore of the following analysis Oreanalysis:

Per cent Crece 44.00 Feo 23.80 SiOa 6.20 A1203 13.80 Mg@ 8.30 Ca@ 3.02

Example l A charge of the chromite ore was smelted in an electricfurnace with lime sumcient to form a self-disintegrating altered ore andcoke sumcient toreduce a portion oi. the displaced ferrous oxide withthe production of a metallic iron product, the altered ore product andthe metallic iron product were obtained in the molten state.

The altered ore product produced was separated from the metallic ironand cooled to permit disintegration. The self-disintegrating altered orewas passed through a mesh screen to separate unaltered ore particles andany relatively large metal particles, and the screened product wastreated in the form of a pulp with Water on a concentrating table toform a tall product and a concentrate. The concentrate was re-treatedwith sulphuric acid to remove additional gangue materials, and a cleanconcentrate analyzing as follows was obtained.

Example II A lcharge consisting of ore and lime and containing the oreand lime in thevproportions, 4

, Per cent Cr203 63.3 FeO 7 3 S102 Nil A1203 llo Ca lg MgO ll.5

About 96.5% of the chromium oxide of the original ore was recovered inthis'product.

Example IH A charge consisting oi two parts by weight of ore and onepart by weight of lime (Cao) `Was ground to approximately D-mesh andsintered at about l350 C. This product did not disintegrate probablybecause of insumcient dicalcium silicate. When ground to 10G-mesh,concentrated by tabling an aqueous pulp of the same and the concentratetreated with acid, it gave a product o the following analysis which con-30 tained 86.5% oi? the chromium oxide of the original ore i Per centCraOs '74.05 FeO '7.35 S1202 1.10 A1203 3.15 Caio MgO 11.15

It will be noted that with the lesser amount of lime less alumina(A1203) was removed, also less soluble Cr203 was formed, vWhile in thecase of more lime, more chromic oxide (Cr203) was rendered soluble, butmore alumina (A1203) was removed, and the product approached a puremagnesium chromite with minor quantities of impurities of silica (SiO2),alumina (A1203), lime (CaO) and ferrous oxide (Feo) In forming thecomposite reagents of the invention, the altered mineral concentrate(oxidized or unoxidized) and the solid non-carbonaceous reducing agentmay be ground separately and mixed together subsequently or the twoproducts may be ground together. I prefer to grind the two productstogether at least in the preparation of the nal composite reagent, asgrinding of the two products together results in thorough mixing andaids in coating oi the larger particles of each product with theextremely ne particles o1- the other product, thus producing the mostdesirable type oi intimate contact. Grinding and mixing eciencies areimproved when the solid noncarbonaceous reducing agent is groundpreliminarily to a finely divided condition. in employing siliconcontaining reducing agents, according to my preferred practice, l employa reducing agent ground initially to such an extent that a iargeproportion by Weight consists ci particles small enough to pass aimi-mesh screen.

En the preferred composite reagents of my invention, the major portions(by Weight) ci the particles or the altered mineral and non-carbonaceous reducing agent combined are small enough to pass 10o-meshscreen. Substantial amounts consist of particles small enough to pass a15o-mesh screen and of particles small enough to pass a 20G-mesh screen.Substantially all oi' the particles may be small enough to pass a20D-mesh screen, and this type of product is most desirable when thecomposite reagent is to be used for the direct production of a metalproduct containing iron and chromium in the proportions in which theseelements are present in the components of the composite reagents. Whenthe composite reagent is to be employed on the surface oi a molten metalbath for incorporating chromium in the metal, such a fine state ofdivision of the components of the composite reagent is not so essentialto the securing oi good results. Such a ne state of division may bedesirable to secure optimum results when the composite reagent is to beemployed on the surface of a slag-covered molten bath. A compositereagent having its components in such a fine state of division reactssubstantially' entirely within itself with a minimum tendency towardreduction of undesirable elements such as phosphorus which may bepresent in the slag.

The products of my invention may b employed in any suitable manner forproducing chromiumbearing alloys. The altered mineral concentrates, forexample, may be added to the furnace either alone or in admixture withthe reducing agent when reduction is to be carried out in an electricfurnace Where ample heat is available. When reduction is to be carriedout in combustion furnaces or in crucibles, the altered mineralconcentrate preferably is mixed intimately with the reducing agentbefore being charged into the furnace or crucible. The intimate mixturemay be employed in a loose condition, or it may be employed in a compactcondition, as, for example, in the form of briquettes, or tightly packedin combustible bags or metal containers. In adding the intimate mixtureof altered mineral concentrate and reducing agent to a combustionfurnace, I prefer to employ it in the form of a compact mass, as in theform of briquettes or agglomerates. y

When the composite reagentsare to be formed into briquettes or packed incontainers, it is desirable to employ particles of diierent sizes in theranges to 100 mesh; 10() to 150 mesh; to 200 mesh; and minus 200 mesh.Substantial amounts of the material will consist oi particles Withinthese sizefranges when grinding is carried out under conditions designedto accomplish reduction in size of about 5 to l0 pei cent. (5 to 10%) ofthe materials to minus 20| mesh particles. The use of various sizedpartil cies provides ior an effective degree of interlock.` ing ofparticles which tends to produce strom briquettes capable ofwithstanding rough han dling in shipping and which also tends to aid ilsecuring the desirable intimate contact of parti cles of altered mineralconcentrate With particle of reducing agent. lhe use of various sizedpar ticles also provides for tight packing of the ma terials incontainers.

in orming composite reagents for use in rele tively low-temperatureoperations, i prefer t subject the aitered mineral concentrates, in tiAsolid state, to an oxidizing treatment to bree down the chromite andmaire the chromiui more amenable to reduction at such loftvtempelatures. Oxidizing treatments may loe carrie I out advantageously in suchcases for the add tional purposes ci" incorporating additional ox:

gen and securing improved exothermiclty and incorporating lime to uxsilica formed during the exothermic reactions.

Suitable oxidation may be eected by roasting the altered mineralconcentrates at temperatures ranging from about 700 C. to 1000* C. inthe presence oi' lime and a little soda ash. The time of roasting mayvary considerably, depending, among other things, on the temperatureemployed, the degree of oxidation desired and the type of equipmentemployed. Oxidation may be carried out in any suitable apparatus. Rotarykiln and riverberatory furnace (with rabbling) treatments producesatisfactory oxidation. Lime may be employed in any suitable amount, hutI prefer amounts not exceeding` those which are capable of entering intochemicalgcombination with the oxides oi the oxidized material, as freelime tends to cause absorption of. moisture and carbon dioxide whichhinder furnace operations in which the oxidized material may beemployed. The oxidized material is :apable of combining chemically withall lime required to ux silicon necessary for reduction )i the reducibleoxides of the material. If it is iesired to employ an excess of lime inaddition o the chemically combined lime, I prefer to add :uch excesslime to the reduction furnace sepa- 'ately and prior to adding theoxidized mate- 'lal.

Oxidation converts ferrous oxide and chromic xide to ferrie oxide andchromium trioxide vhich latter forms chromates with lime and nagnesiapresent in the oxidation charge. The legi-ee and amount of oxidationaccomplished 1an be controlled and will vary in accordance Vithvariations in the technique of procedures n which the composite reagentsare to he ernloyed. The product of the oxidation treatment, rhere onlypartial oxidation of the chromium to hromic anhydride has beenaccomplished may e described as a chromite-chromate product heause ofthe presence therein of both chromite nd chromate.

The following procedures are described to ilistrate the value of thealtered mineral concenrate in the production of chromium steels. Deillsof procedure may vary widely.

Using an altered mineral concentrate produced y one of the methods ofthe invention described cove and having the following analysis:

Per cent m03 74.05 e 7.35 [O2 1.10 no3-; 3.15 'gO 11.15

a0 erst roasted, at '100 o. to 1ooo c., 10o parts the concentrate with100 parts of calcium oxe (CaO) and a little soda ash (l to 5%) ascatalyzer, to produce a 'chromite-chromate A second portion of thechromite concentrate 75 Si was smelted with coke and silicato produce aferrochrome silicon of the following analysis:

Per cent Gr 44.8 Fe, 5.2 Si 50.0

The roasted chromite-chromate product may be mixed with the ferrochromesilicon product in the proportions. parts of roasted product to 32.401parts oi ferrochrome silicon to produce an exothermicmixture forreaction'on a steel bath to produce chromium steel or for reactionseparately to produce a high chromium low carbon metal. In such mixture15% excess silicon is all that is required for eective reduction.

To make 2000 pounds oi 12% chromium steel, a charge of 1726 pounds ofsteel scrapis melted in the usual manner in an electric furnace or openhearth furnace and the carbon reduced with iron ore or mill scale (orthe chromite-chromate product may he used as preferred) to about .06%.There is then charged to the bath with or without rst removing the slagas the conditions may warrant 1110 pounds of the chromate-chromiteferrochrome silicon mixture. After the reaction is over, which is rapidand smooth, the steel is tapped and finished in the :usual manner. It isto be noted that in this case there is sumcient combined lime in thechromite-l chromate to form a limesilicate slag with the silica (S102)from the silicon reduction and produce a slag with a lime to silica(CaO:SiO2) ratio of about 1.6 to 1. There is produced by this methodabout 700 pounds of slag perrton oi' steel. The dimcultly reduciblemagnesium chromite spinel; bythis method, is rendered easily reducibleat lower temperatures.

In a second modification I may use the high chromic oxide (CrnOa)altered mineral concentrate directly with ferrosilicon to producechromium steel. l

Thus, to produce 2000 pounds of 12% chromium steel directly, I melt downin the usual manner 1538 pounds of'steel` scrap and after melting andremoving the carbon to about .06%

with mill scale or iron ore (or the chromitechromate product may be usedif desired), I add 583 pounds of high CrzOa concentrates, 707 pounds oflime (CaO) and'382 pounds of 50% ferrosilicon, and continue to heatuntil practically all the chromic oxide (CrzOa) is reduced by thesilicon. In this case I use up t 50% excess silicon (Si) for reductionbecause of the high temperature required and somewhat less eicientreduction of the chromium spinel. 4In this type of operation, the amounto1 excess silicon will vary widely with conditions of operation,depending to a considerable extent on the reducible components in theslag on the metal bath and the amount of silicon it is desired toincorporate in the metal product.

After the chromic oxide (CrzOa) has been reduced, the steel is tappedand i'lnished inthe usual manner. The slag volume per ton of steel bythis method is about 1200 pounds.

In a third variation in the use of the high chromic oxide (CriOa)altered mineral concentrate, I rst reduce a portion as above stated withcarbon and silica (S102) to produce a ferrochrome silicon of thefollowing analysis 8 To make 2000 pounds of 12% chromium steel, I meltdown, in an electric furnace or open hearth furnace, approxmately 1660pounds of steel scrap free of chromium and reduce the carbon to about.06% with mill scale or iron ore in the usual manner. I then add to thebath 372 pounds of high chromium oxide altered mineral concentrate, 459pounds of lime (CaO) and ,243 pounds of ferrochrome sillcon. This isabout 50% in excess of the silicon (Si) theoretically required, and, asin the second case above, it is used because of the high temperaturerequired and the somewhat less'eflicient reduction of the high chromicoxide (CrzOa) magnesium spinel. After the chromic oxide (CrzOa) isreduced, the steel is tapped and finished in the usual manner. 1n thiscase, 80% recovery of chromium is obtained and the slag volume is about785 pounds.

In carrying out the above-described procedures for producingchromium'steel, about eighty per cent. (80%)- recovery is obtained, butwith careful technique up to ninety-five per cent. (95%) recoveries havebeen obtained. As a general rule more efficiency, shorter time, andlower temperatures are secured by the first method, namely, by roastingto chromite-chromate and flne grinding and intimately mixing to producea self-propagating mixture which reacts within itself to produce moltenchromium metal, than by the other methods where the reaction of siliconwith chromic oxide (CrzOs) is mainly dependent on. the contact of thesilicon in the metal and the chromic oxide (CrzOa) in the slag at thecontact area of the slag and metal. In the first method, thechromite-chromate product (MgO CaO CraOa CrOa) is more readily reducibleand at lower temperatures than the magnesium chro- One of the advantages'of the high chromic oxide (CrzOa) spinel of the altered mineralconcentrate for making chromate-chromite mixture is that sufficient limecan be added without undue dilution and chemically combi ed with thebasic and acidic oxides in the roasting to supply sufficient lime toform a lime-silica (Cao-S102) slag of 1.5 lime to 1 silica or betterwhich is basic enough to give good control of phosphorus and silicon inthe metal `and maintain high exother; micity. The combined lime furtherlessens the danger of hydrogen inclusion in the resulting steel. Anotherimportant advantage of the altered mineral concentrate of the inventionresides in its availability for producing low carbon chromium steelsbecause of its freedom from carbon. Composite reagents and smeltingcharges including the altered mineral concentrate as a component needonly contain the carbon present in the non-carbonaceous reducing agentsand other materials forming other components of the composite reagents.In.l the case of composite reagents, this amount of carbon can be madevery low by proper selection of the non-carbonaceous reducing agents.The amount of carbon in the composite reagents need not exceed about0.0i to 0.04 per cent. by weight.

This application is a continuation-impart of my applications Serial No.244,697, filed Decemloer i938, and Serial No. 252,743, filed Janilary25, i939.

The accompanying flow sheet drawing illustrates the production ofreaction mixtures and the recovery of chromium from chromite ore inaccordance `with the invention.

l claim:

il. The method of recovering chromium which comprises heating chromiteore with lime to form a fused product containing altered or substitutedchromite high in chromic oxide and low in alumina, subsequentlysubjecting the material of the fused product in finely divided form to aconcentration treatment to recover a finely divided mineral concentratecontaining altered or substituted chromite, subjecting the mineralconcentrate to an oxidizing treatment to convert chromium containedtherein to the hexavalent condition, and subsequently reducing to themetallicl state chromium contained in the product of the oxidizingtreatment.

2. The method of producing a reaction mixture suitable for use in theproduction of chromium-bearing alloys which comprises heating chromiteore with lime to form a fused product containing altered or substitutedchromite high in chromic oxide and low in alumina, subsequentlysubjecting the material of the fused product Ain finely divided form toa concentration treatment to recover a finely divided mineralconcentrate containing altered or substituted chromite, subjecting themineral concentrate to an oxidizing treatment to convert chromiumcontained therein to the hexavalent condition, and mixing the product ofthe oxidizing treatment with a non-carbonaceous reducing agent capableof reducing to the metallic state chromium contained therein.

3. The method of treating chromium-bearing material containing spineltype mineral comprising ferrous oxide, alumina and chromic oxide whichcomprises subjecting the material to a fusion treatment under reducingconditions and under the influence of a basic compound to effecialteration of the spinel typemineral and reduction of ferrous oxide withthe production of s metallic iron product low in chromium and th:production of a non-metallic product containlng an artificial spineltype mineral comprising 'chromic oxide, ferrous oxide and alumina 'irwhich the ferrous oxide and alumina are presen in proportions,relatively to chromic oxide, small er than in the spinel type mineral ofthe origi nal chromium-bearing material, separating thi metallic ironproduct and the non-metallic prod uct, subjecting the non-metallicproduct in fine ly divided form to a concentration treatmen to recover afinely divided mineral concentrat containing the artificial spinel typemineral, sub jecting the mineral concentrate to an oxidizin treatment toconvert chromium contained there in to the hexavalent condition, andmixing th product of the oxidizing treatment with a non carbonaceousreducing agent capable of reducin to the metallic state chromiumcontained thereil A composite reagent suitable for use in th productionof chromium alloys which comprise a mixture of (l) a solid, finelydivided proff uct consisting essentially of artificial spinel ty;mineral containing chromic oxide produced t fusing chromite ore withlime and subjectirI the product of the fusion treatment to a concertration treatment and (2) a finely divided, soll non-carbonaceousreducing agent capable of re ducing chromic oxide.

5. A composite reagent suitable for use in tl production of chromiumalloys which compris a mixture of (l) a solid, finely divided produconsisting essentially of artificial spinel type mii eral containingchromic oxide in an amount n less than about fifty percent by weightproducf` by fusing chromite ore with lime and subjectli the product ofthe fusion' treatment to a conca tration treatment and (2) a iinelydivided, solid, non-carbonaceous reducing agent capable of reducingchromic oxide.

6. A composite reagent, suitable for use in the production ofchromium-bearing alloys, in the form of an intimate mixture of solid,nely divided components comprising (1) one or more compounds of thegroup consisting of calcium chromite and magnesium chromite, (2) one ormore compounds of the group consisting of calcium chromate and magnesiumchromate produced by fusing chromite ore with lime, subjecting theproduct of the fusion treatment to a concentration treatment to recovera chromiumbearing concentrate, and subjecting the concentrate to anoxidation treatment to oxide chromium contained therein to thehexavalent condition, and (3) ferrochrome silicon in which the ratio ofchromium to iron by weight is not less than about three to one.

7. A composite reagent, suitable for use in the production ofchromium-bearing alloys, in the form of an intimate mixture of solid,iinely divided components comprising (l) one or more compounds of thegroup consisting of calcium chromite and magnesium chromite, (2) one ormore compounds of the group consisting of cal-- cium chromate andmagnesium chromate produced by fusing chromite ore with lime, subjectingthe product of the fusion treatment to a concentration treatment torecover a chromiumbearing concentrate, and subjecting the concentrate toan oxidation treatment to oxidize chromium contained therein to thehexavalent condition, and (3) ferrochrome silicon in which the ratio ofchromium to iron by weight is not less than about three to one, thecalcium and magnesium being present in said mixture in amount at leastatomically equivalent to the silicon contained therein, and said mixturebeing substantially free of uncombined lime and magnesia.

MARVIN J. UDY,

